1. Field of the Invention
The present invention relates to a beam adjusting sample, a beam adjusting method and a sample adjusting device.
2. Description of the Related Art
Conventionally, an electron beam radiating apparatus for applying an electron beam to an object and detecting the shape of the object has been well known. As the electron beam radiating apparatus of this type, a scan type electron microscope (SEM) is well known as disclosed in JP-A-5-159735 (page 2, FIG. 5).
In the scan type electron microscope, a predetermined voltage is applied between a cathode and an anode, electrons flying out of the cathode are axially adjusted by an alignment coil, converged by a focusing lens, and focused via a stigma coil, an alternation coil and an objective lens to a predetermined position of the sample, and secondary electrons generated from the sample are detected by a secondary electron detector.
Referring to FIGS. 1 and 2, an electron beam adjusting method in the electron beam radiating apparatus of this type will be described below.
First of all, an electron beam 100 is applied onto a microstructure including a plurality of latex balls of known size laid on a stage and roughly adjusted, as shown in FIG. 1. At this time, the electron beam 100 is coarsely focused on the microstructure by changing the power of the objective lens in the electron beam radiating apparatus while seeing an image produced based on the detected secondary electrons.
If the focal point of the electron beam 100 is adjusted approximately on the latex balls 110, the beam diameters of electron beam in the x and y directions are adjusted to make the spot shape of electron beam almost circular. FIGS. 2 and 3 are views for explaining the beam diameter adjustment using a cross wire. In FIG. 2, two wires 120 and 130 crossed at right angles and a Faraday cup 140 placed directly under the wires 120 and 130 are shown.
The wires 120 and 130 are made of tungsten, for example, with the thickness being about 30 μm. The wires 120 and 130 extend in the x and y directions orthogonal to each other. In the adjustment using this cross wire, the electron beam 100 is scanned to stride the wires 120 and 130 along the x and y directions, and an amount of electron beam incident upon the Faraday cup 140 disposed under the crosswire is converted into an electrical signal. The electrical signal is amplified by an amplifier 150, passed through a low pass filter 160 for the wave form processing, and displayed on an oscilloscope 170.
FIGS. 4A to 4C are typical views showing the relationship between the electron beam 100 incident on the Faraday cup 140 and the wires 120 and 130. For simplification, a wire 120 is taken as an example here. As shown in FIG. 4A, if the electron beam 100 is totally applied over the wire 120, the electron beam 100 is obstructed by the wire 120, and does not enter the Faraday cup. This state is defined as state A. Next, if the electron beam 100 is applied near an edge of the wire 120, a partial electron beam 101.passes by the wire 120 to enter the Faraday cup, while a remaining electron beam 102 is scattered by the wire 120 to produce scattered electrons 103. This state is defined as state B. Moreover, when the electron beam 100 is not applied to the wire 120, but totally passes by, all the electron beam 100 is incident on the Faraday cup 140. This state is defined as state C.
FIG. 5 is a graph showing an output waveform of the Faraday cup 140 when one of the wires 120 and 130 is continuously scanned using the electron beam 100. In the graph of FIG. 5, the axis of abscissas is the beam scan position, and the axis of ordinates is the output of the Faraday cup 140, in which “A”, “B” and “C” on the axis of abscissas correspond to the states as shown in FIGS. 4A to 4C. As shown in FIG. 5, the output of the Faraday cup 140 rapidly rises from state A to state C. In the adjustment, the beam profile is assumed to be Gaussian, and the width between the beam position at which the output of the Faraday cup 140 is 12% and the beam position at which it is 88% is defined as a beam diameter. The adjustment is made in each of the x and y directions to minimize the beam diameter and make the beam diameter almost equal to produce an almost circular spot electron beam with high precision.
However, with the above method, when the coarse adjustment using the latex balls is insufficient, it is required to repeat the adjustment many times to continue the operation, until an excellent beam diameter is obtained, which takes a quite enormous time. Also, this adjustment needs some experience, and is not treated simply.
With the cross wire method, though two wires are employed to measure the beam diameters in the x and y directions, the measurement height is different in the adjustment in the x and y directions by an amount of wire width, because the wire has a width, as shown in FIG. 3. Accordingly, the correct height position of the cross wire is not accurately measured, resulting in an error in the measurement precision.
Also, in the state B, the partial electron beam 100 is scattered as the scattered electrons 103, but part of scattered electrons 103 are scattered by the wire 120 to enter the Faraday cup 140, as shown in FIG. 4B. Accordingly, an output curve of the Faraday cup as shown in FIG. 5 is raised by part of scattered electrons and does not represent the beam profile accurately. Accordingly, it is difficult to make the measurement at high precision.
Also, the wires 120 and 130 are partially varied in the shape, the height in the z axis direction being possibly different depending on the position, whereby the measurement precision is degraded due to this height error.